“Click” and “pop” are terms used to describe unwanted audio-band transient signals that are heard in a headphone or a speaker when an audio amplifier is enabled.
In portable audio applications power consumption is a key issue, which means that circuit components, such as audio amplifiers, are often disabled or powered down when not required. This can lead to unwanted audio-band transient signals being produced, both when an audio amplifier is powered down or placed in a sleep or hibernation mode, and when an audio amplifier is powered up or enabled from a sleep or hibernation mode. Similar problems can also arise in other non-portable applications.
Click and pop problems are particularly problematic in single supply amplifiers that have to charge to a certain defined voltage during power-up.
FIG. 1 shows a known audio amplifier circuit 1 for driving a load 2, for example a headphone or a speaker, coupled to an output terminal 3. An output amplifier 5 receives an audio signal at a first input terminal 7 from an audio source, such as a mixer 9. It will be appreciated that the mixer 9 receives an audio signal from a DAC (not shown) or other signal source. The amplifier 5 also receives a reference voltage VMID at a second input terminal 11. In order for the output signal of the amplifier to achieve maximum swing, either side of its quiescent output voltage, this quiescent voltage is set midway between the supply voltages VDD and ground (GND). The quiescent voltage is set be an applied reference voltage VMID, equal to VDD/2.
The reference voltage VMID is produced by a reference voltage generator circuit 13. As will be described in greater detail below, a transient signal may be produced when the reference voltage generator circuit 13 is powered up, thereby causing an unwanted “pop” being transmitted to the headphone or speaker.
It is noted that control logic 10 is provided for controlling the operation of the output amplifier 5 during power up and mute operations. For example, the control logic 10 provides a control signal S1 for controlling the reference generator circuit 13, a control signal S2 for controlling the amplifier 5 (for example when performing a mute operation), and a control signal S3 for controlling a buffer circuit 14. The buffer circuit 14 buffers the reference voltage VMID received from the reference voltage generator circuit 13. It is noted that the buffer circuit 14 is not essential to the operation of the amplifier circuit.
Referring to the flow chart of FIG. 2, a brief description of a typical power-up sequence is provided. A similar sequence of operations will occur when the amplifier is re-enabled (i.e. enabled) after a, period of being disabled (i.e. after hibernation). On initial application of power, step 201, the signal path from input to output is in a mute state, i.e. in a state where the output is unaffected by the input signal, for example by interrupting the signal path using a switch. The amplifier 5 is in a disabled state, i.e. not driving its output.
The reference voltage generator circuit 13 that produces the reference voltage VMID is then enabled, step 203. This is performed, for example, by closing the switch 131 of FIG. 1. There is a delay while the reference voltage stabilises, and while the decoupling and AC coupling capacitors charge, step 205. This delay can take approximately 1 second based on total capacitive load. It is noted that the AC coupling capacitor 15 may be charged, for example, using a bypass signal path having a bypass switch 17 as shown in FIG. 1. This allows the reference voltage VMID to bypass the disabled amplifier 5 and charge the AC coupling capacitor 15 to VMID.
Once the reference voltage VMID has settled the output amplifier 5 is enabled, step 207. The amplifier 5 is then un-muted, step 209, thereby connecting the amplified audio signal to the output terminal 3.
Since the reference voltage VMID is connected to the load 2, via bypass switch 17, when the reference voltage generator circuit 13 is being enabled a “pop” is produced due to a slope discontinuity, i.e. rapid deviation or change, in the rate of change of the reference voltage VMID across the capacitor 135. The slope discontinuity produces audible signal components that propagate through to capacitor 15 and onto the load 2, thereby causing an audible click or pop.
FIG. 3 shows a typical reference voltage generator circuit 13 for producing the reference voltage VMID. The reference voltage VMID can be produced using a potential divider circuit, for example, that comprises resistive elements 137 and 139. If the voltage level of the reference voltage is chosen to be VDD/2, then the resistive elements 137 and 139 will have equal values. It will be appreciated that the resistive elements 137 and 139 would have different values if a different reference voltage was required. A decoupling capacitor 135 is connected across resistive element 139. It is noted that, in the case of an integrated circuit arrangement, the decoupling capacitor 135 may be provided off-chip, if desired, and is used to decouple the VMID node 133. A switch 131 is provided for enabling and disabling the reference voltage generator circuit 13 under control of the control signal S1.
FIG. 4 shows the VMID voltage at node 133 during power-up of the reference voltage generator circuit 13 when the audio amplifier is enabled or powered-up. Before the reference voltage generator circuit 13 is switched on at tON, the decoupling capacitor 135 is effectively short-circuited to ground via resistor 139. When the reference voltage generator circuit 13 is switched on at tON, this results in a rapid deviation or change in the reference voltage VMID across the capacitor 135. As the decoupling capacitor 135 continues to charge, the rise in the voltage VMID becomes more gradual until the desired reference voltage VMID is reached. This slope discontinuity of the reference voltage VMID at tON is what causes the audible pop.
One method of avoiding these slope discontinuities would be to increase the value of resistor 139. However, an increased value of resistor 139 would lead to an unacceptably long charge time (e.g. 5 to 10 seconds), whereas the charge time is desired to be a few hundred milliseconds
Due to the above mentioned click and pop problems, it is therefore an aim of the present invention to provide an apparatus and method for reducing unwanted signals in an audio circuit.